1. Field of the Invention
The present invention relates to a technology for a method for producing a non-aqueous electrolyte secondary battery, a non-aqueous electrolyte secondary battery, and a method for producing a negative electrode paste.
2. Description of Related Art
Non-aqueous electrolyte secondary batteries, such as lithium-ion batteries, for use in hybrid vehicles are required to have excellent output characteristics and cycle characteristics. Conventionally, various techniques have been examined for establishing a simple method for preparing a high-quality paste for electrodes in order to improve output characteristics or cycle characteristics. For example, a related art is disclosed in Japanese Patent Application Publication No. 2011-63673 (JP 2011-63673 A) that is shown below.
JP 2011-63673 A enables an aqueous paste for electrodes to be produced simply by adding a solvent to a mixture of powdery active material particles and carboxymethyl cellulose (CMC).
However, when a non-aqueous electrolyte secondary battery is produced using a paste that is produced according to the related art that is disclosed in JP 2011-63673 A, defects (i.e., defect points) that are derived from CMC may occur in the composite material layer of the negative electrode depending on the physical properties of the negative-electrode active material or some other reasons.
For example, it is effective to reduce the additive amount of CMC that is contained in the negative electrode to produce a high-output non-aqueous electrolyte secondary battery. It is therefore effective to use CMC with a high viscosity (in other words, with a high molecular weight) to reduce the additive amount of CMC. However, CMC with a high viscosity is poorly soluble. When CMC is not dissolved completely during kneading, undissolved CMC may remain in the negative electrode paste and cause defects during coating. Thus, when CMC with a high viscosity is used to improve the output characteristics, the problem of the tendency for defects to occur during coating needs to be solved.
It is known that the solubility of CMC also depends on the amount of oil that has been adsorbed on the negative-electrode active material. For example, when the amount of oil that has been adsorbed on the negative-electrode active material is insufficient, the solid content rate becomes too high (in other words, the amount of water becomes insufficient) during hard kneading to dissolve CMC completely. On the other hand, when the amount of oil that has been adsorbed on the negative-electrode active material is excessive, the adsorption of CMC on the negative-electrode active material increases, resulting in poor peeling strength of the composite material layer of the negative electrode. Then, another problem, the tendency for the cycle characteristics to deteriorate, occurs.
Thus, when CMC with a high viscosity is used in preparing a negative electrode paste, it is necessary to optimize the relationship between the viscosity of CMC and the amount of oil that is adsorbed on the negative-electrode active material in order to dissolve CMC completely and to ensure sufficient peeling strength of the composite material layer.